There is mounting evidence that testosterone acts to maintain spermato-genesis through the interaction of this steroid with Sertoli cells. Thus, physiological situations which lead to a diminution of androgen concentration around Sertoli cells, or which lead to a reduction in the ability of these cells to respond to testosterone, could lead to a reduced production of testicular spermatozoa. In the male rat, testicular aging is characterized by a decline in sperm production and by a decrease in testosterone production by Leydig cells. We suggest that in the aged rat, decreased T production leads to decreased intratesticular testosterone concentration, and therefore, to diminished Sertoli cell function. While the consequences to spermatogenic cells of a hypofunctional Sertoli cells and a reduced number, or possibly reduced function, of stem spermatogonia. The experiments outlined in this application will test this hypothesis. First, wee will determine the effect of age and different intratesticular testosterone concentrations on the number and functions of rat Sertoli cells. Next, we will examine the effect of age on the number and rate of cell division of stem spermatogonia. Finally, we will examine how different intratesticular testosterone concentrations affect the number and function of stem spermatogonia in young and old rats.